1. Field of the Invention
The present invention relates to supported catalysts containing a platinum metal and their use in processes for preparing diaryl carbonates by reaction of aromatic hydroxy compounds with carbon monoxide and oxygen, which are characterized in that the supported catalysts contain at least one cocatalyst in addition to the platinum metal.
2. Description of the Related Art
It is known that organic carbonates can be prepared by oxidative reaction of aromatic hydroxy compounds with carbon monoxide in the presence of a noble metal catalyst (German Offenlegungsschrift 28 15 512). The noble metal preferably used is palladium. In addition, a cocatalyst (e.g. manganese or cobalt salts), a base, a quaternary salt, various quinones or hydroquinones and desiccants can be used. The reaction can be carried out in a solvent, preferably methylene chloride.
For economically carrying out this process, not only the activity and the selectivity but also the effective recovery of the noble metal catalyst are of decisive importance: for one thing, the noble metal catalyst represents a considerable cost factor. Losses of noble metal catalyst have to be replaced at high cost. Furthermore, no residues of noble metal catalyst may remain in the product. For the process of oxidative carbonylation of aromatic hydroxy compounds to give diaryl carbonates, the economical and efficient recovery of homogeneous catalysts has hitherto not been described. A noble metal catalyst can be separated from a liquid reaction mixture with little effort, e.g. by filtration or centrifugation, if heterogeneous catalysts, e.g. supported catalysts, are used.
For preparing supported catalysts, suitable materials are known. Depending on the type of process, use is made of supports having a high internal surface area, for example aluminium oxide, magnesium oxide, activated carbon or silicon dioxide having more than 50 m2 of surface area per gram, supports having surface areas around 5 m2/g and correspondingly larger pore radii, for example carbon black, titanium dioxide, iron oxide or zinc oxide, or coarse-grained supports, for example silicon carbide and corundum (Ullmanns Enzyklopxc3xa4die der technischen Chemie, 3rd edition, Berlin/Munich 1957, Volume 9, p. 263 ff). Essentially, it is possible to use both synthetic materials such as activated aluminium oxides, silica gels, silicates, titanium dioxides or activated carbons and also materials from natural sources, for example pumice, kaolin, bleaching earths, bauxites, bentonites, kieselguhr, asbestos or zeolites. In EP 572 980, EP 503 581 and EP 614 876, use is made of supported noble metal catalysts containing 5% of palladium on carbon supports. However, according to our own studies, such supported catalysts give only very unsatisfactory conversions, if any, so that these too are not suitable for an economical process. In JP 01/165 551 (cited according to C.A. 112 (1990), 76618j) it is stated that, for the preparation of aromatic carbonates, palladium or palladium compounds such as palladium acetylacetonate can be used in combination with alkali metal (alkaline earth metal) iodides or onium iodides such as tetrabutylammonium iodide and at least one zeolite. JP 04/257 546 and JP 04/261 142 describe, in one example each, a supported catalyst for preparing aromatic carbonates in which granulated silicon carbide is used as support material for a supported catalyst in a distillation column. Although the relevant examples are carried out under drastic conditions (high pressure, high temperature), this catalyst makes possible only very low space-time yields. These low space-time yields make economical preparation of aromatic carbonates using such supported catalysts impossible.
Up to now, there is therefore no supported catalyst available by means of which diaryl carbonates can be prepared economically and efficiently by reaction of an aromatic hydroxy compound with carbon monoxide and oxygen. It was therefore an object of the invention to find a supported catalyst having high activity and selectivity which allows the economically efficient preparation of diaryl carbonates by reaction of an aromatic hydroxy compound with carbon monoxide and oxygen.
It has now been found that the above disadvantages can be overcome if use is made of supported platinum metal catalysts which contain at least one cocatalyst in addition to the platinum metal. The catalysts comprising platinum metal and at least one cocatalyst on a support are used according to the invention as powders, pellets or binder-containing extrudates. Suitable binders are, for example, SiO2, Al2O3 or clay minerals. The binder contents can be varied within a wide range, for example from 0.5 to 99.5% by weight, based on the total weight of the support.
The supported catalysts of the invention contain, in the reaction-ready state, (i) a platinum metal, a platinum metal halide or a complex containing a platinum metal halide, or a compound which can be converted under the reaction conditions into a platinum metal, a platinum metal halide or a complex containing a platinum metal halide, in an amount of 0.01-15% by weight, preferably 0.05-10% by weight, calculated as platinum metal and based on the total weight of the catalyst, and (ii) a metal compound acting as cocatalyst from groups IB, IIB, IIIA, IIIB, IVA, IVB, VB, VIB, VIIB, the iron group (atomic numbers 26-28) or the rare earth metals (atomic numbers 58-71) of the Periodic Table of the Elements (Mendeleev) in an amount of 0.01-15% by weight, preferably 0.05-10% by weight, calculated as metal and based on the total weight of the catalyst.
Such catalysts are in the form of heterogeneously catalytic systems and thus make it easier to separate the reaction product from the expensive platinum metal, its compounds and the cocatalyst.
The invention further provides a process for preparing an aromatic carbonate of the formula
Rxe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94Rxe2x80x83xe2x80x83(I),
where
R is substituted or unsubstituted C6-C12-aryl, preferably substituted or unsubstituted phenyl, particularly preferably unsubstituted phenyl,
by reaction of an aromatic hydroxy compound of the formula
Rxe2x80x94Oxe2x80x94Hxe2x80x83xe2x80x83(II),
where R is as defined above,
with carbon monoxide and oxygen in the presence of a quaternary ammonium or phosphonium salt and a base at from 30 to 200xc2x0 C., preferably 30-150xc2x0 C., particularly preferably from 40 to 120xc2x0 C., and at a pressure of from 1 to 150 bar, preferably 2-50 bar, particularly preferably from 5 to 25 bar, which is characterized in that use is made of supported platinum metal catalysts which contain one or more cocatalysts of the above-described type and amount in addition to the platinum metal.
For the example of the formation of diphenyl carbonate, the process of the invention can be represented in terms of formulae as follows:
2C6H5xe2x80x94OH+CO+xc2xdO2xe2x86x92(C6H5O)2CO+H2O
Catalyst supports suitable for use according to the invention are all industrially customary catalyst supports, for example those based on carbon, element oxides, element carbides or element salts in various use forms. Examples of carbon-containing supports are coke, graphite, carbon black or activated carbon. Examples of element oxide catalyst supports are SiO2 (natural or synthetic silicas, quartz), Al2O3 (xcex1-, xcex3-Al2O3), aluminas, natural and synthetic aluminosilicates (zeolites), TiO2 (rutile, anatase), ZrO2 or ZnO. Examples of element carbides and salts are SiC, AlPO4, BaSO4, CaCO3, etc. They can be used either as chemically uniform pure substances or in admixture. Catalyst supports suitable for use according to the invention can be either in the form of pieces or as pulverulent materials. If the supported catalyst is arranged in a fixed bed, the support is preferably used as shaped bodies, e.g. as spheres, cylinders, rods, hollow cylinders, rings, etc. If desired, catalyst supports can be further modified by extrusion, pelletizing, optionally with the mixing in of further catalyst supports or binders such as SiO2 or Al2O3, and calcination. Preparation and further processing of the catalyst supports used according to the invention are well known to those skilled in the art and are prior art.
The reactive component of the catalyst comprises, in the reaction-ready state, a platinum metal, a platinum metal halide such as PdCl2 or PdBr2, or a complex containing a platinum metal halide, where the said complex can additionally contain, for example, olefins, amines, phosphines, nitriles, carbon monoxide or water, for example A2(PdHal4), where A represents, for example, Li, Na, K, NH4, Rb, Cs or NR14 and R1 represents an organic radical C6-C10-aryl, C7-C12-aralkyl and/or C1-C20-alkyl and Hal represents a halogen such as, for example, F, Cl, Br, I, and also at least one cocatalyst. Suitable platinum metal complexes are basically known. Examples are: Li2(PdCl4), Na2(PdCl4), K2(PdCl4), (NBu4)2(PdCl4), Na2(PdBr4), K2(PdBr4), (NBu4)2(PdBr4) where Bu=n-butyl; examples of olefin-containing platinum metal complexes are [allylpalladium chloride] dimerxe2x80x94[C3H5PdCl]2, 1,5-cyclooctadienepalladium dichloridexe2x80x94C8H5PdCl2; examples of phosphine-containing platinum metal complexes are [1,2-bis(diphenylphosphino)ethane]palladium dichloridexe2x80x94Pd[(C6H5)2PCH2CH2P(C6H5)2]Cl2, bis(triphenylphosphine)palladium dichloridexe2x80x94Pd[P(C6H5)3]2Cl2; examples of amine-containing platinum metal complexes are diamminepalladium dibromide xe2x80x94Pd(NH3)2Br2, diamminepalladium dichloridexe2x80x94Pd(NH3)2Cl2, tetramminepalladium tetrachloropalladatexe2x80x94[Pd(NH3)4][PdCl4]; examples of nitrile-containing platinum metal complexes are bis(acetonitrile)palladium dichloridexe2x80x94Pd(CH3CN)2Cl2, bis(benzonitrile)palladium dichloridexe2x80x94Pd(C6H5CN)2Cl2; examples of carbon monoxide-containing platinum metal complexes are tetrabutylammonium tribromocarbonylpalladatexe2x80x94(NBu4)Pd(CO)Br3 (where Bu=n-butyl) and tetrabutylammonium trichlorocarbonylpalladatexe2x80x94(NBu4)Pd(CO)Cl3 (where Bu=n-butyl). In the examples mentioned, Pd has been specified as platinum metal, but other platinum metals are also suitable, for example Pt, Ir, Ru or Rh. However, Pd and Rh, in particular Pd, are preferred. The platinum metal is present in an oxidation state of from 0 to 4.
It has also been found that the platinum metal halide or the complex containing the platinum metal halide can be prepared in situ on the support during the preparation or during use of the catalyst under reaction conditions from a suitable halogen-free platinum metal compound and a halide-containing compound. Suitable halogen-free platinum metal compounds are, for example, platinum metal nitrates, acetates, propionates, butyrates, oxalates, carbonates, oxides, hydroxides, acetylacetonates and others with which those skilled in the art are familiar. Suitable halide-containing compounds are halogen-containing salts and complexes of the elements of the first to fifth main groups and the first to eighth transition groups of the Periodic Table of the Elements (Mendeleev) and also of the rare earth metals (atomic numbers 58-71), or aliphatic halogenated hydrocarbons. Examples are NaBr, NaCl, MgCl2, MgBr2, AlCl3, CH2Cl2, NaPF6, MnCl2, MnBr2, CoBr2, CeCl3, SmI2, CuCl2, Na2ZnCl4, TiCl4 and NR14 Br, where R1 is as defined above.
The amount of the platinum metal, platinum metal halide or of the complex containing the platinum metal halide in the reaction-ready state is from 0.01 to 15% by weight, preferably from 0.05 to 10% by weight, calculated as platinum metal and based on the total weight of the catalyst.
The cocatalyst used for the process of the invention is a metal compound of group I B, II B, III A, III B, IV A, IV B, V B, VI B, VII B, the iron group (atomic numbers 26-28) or the rare earth metals (atomic numbers 58-71) of the Periodic Table of the Elements (Mendeleev), preferably Mn, Cu, Co, V, Nb, W, Zn, Ce or Mo, particularly preferably Mn, Co, Cu, Mo or Ce. The metals can be used, for example, as halide, oxides, carboxylates of C2-C6-carboxylic acids, diketonates or nitrates and as complexes which can contain, for example, carbon monoxide, olefins, amines, nitriles, phosphines and halides.
The amount of compound containing the cocatalyst in the reaction-ready state is from 0.01 to 15% by weight, preferably from 0.05 to 10% by weight, calculated as metal and based on the total weight of the catalyst.
Platinum metal and cocatalyst can be applied to the support simultaneously, i.e. from a joint solution, or successively in any order. Suitable solvents for the platinum metal and cocatalyst compounds for preparing supported catalysts according to the invention are, for example, water, aliphatic hydrocarbons such as pentane, n-hexane, cyclohexane, etc., aliphatic halogenated hydrocarbons such as dichloromethane, trichloromethane, etc., unsaturated hydrocarbons such as pentene, isoprene, cyclopentadiene, hexenes, hexines, cyclohexenes, cyclooctadienes, etc., aromatic hydrocarbons such as benzene, toluene, xylene, etc., halogenated aromatic hydrocarbons such as chlorobenzene, dichlorobenzene, etc., primary, secondary or tertiary alcohols such as methanol, ethanol, n-propanol, iso-propanol, n-butanol, iso-butanol, t-butanol, cumyl alcohol, iso-amyl alcohol, diethylene glycol, etc., ketones such as acetone, 2-butanone, methyl isobutyl ketone, acetylacetone, etc., ethers such as diethyl ether, diisopropyl ether, methyl t-butyl ether, dioxane, tetrahydrofuran, etc., esters such as methyl acetate, ethyl acetate, etc., nitriles such as acetonitrile, benzonitrile, etc., carbonates such as dimethyl carbonate, diethyl carbonate, diphenyl carbonate, etc., amides such as dimethylacetamide, N-methylpyrrolidinone and tetramethylurea. Of course, mixtures of such solvents can also be used.
The catalysts to be used according to the invention are prepared by methods which are basically known to those skilled in the art. Thus, solutions of one or more of the platinum metal compounds specified and the halide-containing compounds specified and also one or more cocatalysts can be applied, for example by soaking, adsorption, dipping, spraying, impregnation and ion exchange, to the catalyst support to be used according to the invention. It is also possible to fix one or more platinum metals, the halide-containing compounds specified and also one or more cocatalysts on the support by precitation with a base. Suitable bases are, for example, alkali metal (alkaline earth metal) hydroxides such as Ca(OH)2, Mg(OH)2, NaOH, LiOH and KOH, alkali metal (alkaline earth metal) hydrogen carbonates such as Ca(HCO3)2, Mg(HCO3)2, NaHCO3, LiHCO3 and KHCO3, alkali metal (alkaline earth metal) carbonates such as CaCO3, MgCO3, Na2CO3, Li2CO3 and K2CO3, alkali metal salts of weak organic acids, such as sodium acetate, potassium acetate and lithium acetate, and alkali metal (alkaline earth metal) salts of substituted or unsubstituted phenols (in the case of substituted phenols, these salts are those as are described further below as being usable in the process for preparing diaryl carbonate), such as lithium phenoxide, sodium phenoxide, sodium cresoxide and potassium phenoxide. The platinum metal and the halide-containing compound can be applied to the support either successively in any order or simultaneously. A specific embodiment of the invention comprises the application of the platinum metal by precipitation of a platinum metal halide or a platinum metal halide complex with a suitable base (suitable bases are, for example, those as are described above), reduction of the precipitated platinum metal base to the metal using a suitable reducing agent such as, for example, hydrazine, formaldehyde, sodium formate, NaBH4 at temperatures between 0xc2x0 C. and 200xc2x0 C. or gaseous hydrogen at temperatures between 0xc2x0 C. and 500xc2x0 C., preferably between 20 and 300xc2x0 C., particularly preferably 30-250xc2x0 C., and reaction of the platinum metal with hydrogen halide or gaseous halogen at temperatures between 20xc2x0 C. and 600xc2x0 C., preferably between 50 and 500xc2x0 C.
During the application of platinum metal and cocatalyst to the support, the mixture can be stirred. However, it can also be advantageous to allow the mixture to stand or to shake it, so that shaped bodies, if used, are not damaged by a stirrer. After application of platinum metal and cocatalyst to the support, the supported catalyst is separated off, for example, by filtration, sedimentation or centrifugation. In a further embodiment of the invention, the solvent is separated off by distillation.
After separating off the solvent, the supported catalysts thus obtained are dried. This can be carried out in air, in vacuo or in a stream of gas. Suitable gases for drying the supported catalyst in a stream of gas are nitrogen, oxygen, carbon monoxide, carbon dioxide and noble gases and also any mixtures of the gases specified, preferably, for example, air. Likewise suitable are gaseous hydrocarbons such as alkanes (e.g. methane, ethane, propane), alkenes such as ethene, propene, butene, butadiene and alkines such as ethine, propine, etc, in any composition. Drying is carried out at from 20 to 200xc2x0 C., preferably at from 40 to 180xc2x0 C., particularly preferably at from 60 to 150xc2x0 C. The drying time depends, for example, on the porosity of the support used and on the solvent used. It is generally a few hours, for example from 0.5 to 50 hours, preferably from 1 to 40 hours, particularly preferably from 1 to 30 hours.
After drying, the dried supported catalysts can be calcined. This can be carried out in air, in vacuo or in a stream of gas. Suitable gases for calcination of the supported catalyst in a stream of gas are, for example, nitrogen, oxygen, carbon dioxide or noble gases and also any mixtures of the gases specified, preferably, for example, air. Calcination is carried out at from 100 to 800xc2x0 C., preferably from 100 to 700xc2x0 C., particularly preferably at from 100 to 600xc2x0 C. It may here be advantageous if the composition of the gas is changed abruptly or continuously during the calcination. An abrupt change of the calcination gas composition can be carried out, for example, by after 10 hours increasing the O2 content from 10% by volume to 20% by volume for the remaining 10 hours and maintaining the temperature. A continuous change of the calcination gas composition can be carried out, for example, by maintaining the temperature and increasing the oxygen content from 0% by volume to 20% by volume over 20 hours at a rate of 1% by volume/h. The calcination time is generally a few hours, for example from 0.5 to 50 hours, preferably from 1 to 40 hours, particularly preferably from 1 to 30 hours.
The aromatic hydroxy compounds which can be reacted using the supported catalysts of the invention are, for example, phenol, o-, m- or p-cresol, o-, m- or p-chlorophenol, o-, m- or p-ethylphenol, o-, m- or p-propylphenol, o-, m- or p-methoxyphenol, 2,6-dimethylphenol, 2,4-dimethylphenol, 3,4-dimethylphenol, 1-naphthol, 2-naphthol and bisphenol A, preferably phenol. If the aromatic hydroxy compound is substituted, there are generally 1 or 2 substituents which are C1-C4-alkyl, C1-C4-alkoxy, fluorine, chlorine or bromine.
For the process of the invention, any organic or inorganic bases or mixtures thereof can be used. Examples of inorganic bases which may be mentioned are, without restricting the process of the invention, alkali metal hydroxides and carbonates, carboxylates or other salts of weak acids and also alkali metal salts of aromatic hydroxy compounds of the formula (II), e.g. alkali metal phenoxides. Of course, it is also possible to use the hydrates of alkali metal phenoxides in the process of the invention. An example of such a hydrate which may be mentioned here, without restricting the process of the invention, is sodium phenoxide trihydrate. However, the amount of water added is preferably such that a maximum of 5 mol of water are used per mol of base. Higher water concentrations lead, in general to poorer conversions and decomposition of the carbonates formed. Organic bases which may be mentioned, without restricting the process of the invention, are tertiary amines which can bear C6-C10-aryl, C7-C12-aralkyl and/or C1-C20-alkyl radicals as organic radicals or are pyridine bases or hydrogenated pyridine bases, for example triethylamine, tripropylamine, tributylamine, trioctylamine, benzyldimethylamine, dioctylbenzylamine, dimethylphenethylamine, 1-dimethylamino-2-phenylpropane, pyridine, N-methylpiperidine, 1,2,2,6,6-pentamethylpiperidine. The base used is preferably an alkali metal salt of an aromatic hydroxy compound, particularly preferably an alkali metal salt of the aromatic hydroxy compound (II) which is also to be reacted to form the organic carbonate. These alkali metal salts can be lithium, sodium, potassium, rubidium or caesium salts. Preference is given to using lithium, sodium and potassium phenoxide, particularly preferably sodium phenoxide.
The base can be added to the reaction mixture as a pure compound in solid form or as a melt. In a further embodiment of the invention, the base is added to the reaction mixture as a solution containing from 0.1 to 80% by weight, preferably from 0.5 to 65% by weight, particularly preferably from 1 to 50% by weight, of the base. Solvents which can be used here are alcohols or phenols, such as the phenol (II) to be reacted, or inert solvents. Examples which may be mentioned are those mentioned further below as reaction media. These solvents can be used alone or in any combination with one another. Thus, one embodiment of the process of the invention comprises, for example, dissolving the base in a phenol melt which has been diluted with a solvent. The base is preferably dissolved in the melt of an aromatic hydroxy compound, particularly preferably in a melt of the aromatic hydroxy compound (II) which is to be reacted to form the organic carbonate. Very particularly preferably, the base is added in solution in phenol.
The base is added in an amount which is independent of the stoichiometry. The ratio of the platinum metal, e.g. palladium, to the base is preferably selected such that from 0.1 to 500, preferably from 0.3 to 200, particularly preferably from 0.9 to 130, equivalents of base are used per mol of platinum metal, e.g. palladium.
The process of the invention is preferably carried out without solvent. Of course, inert solvents can also be used. Examples of solvents which may be mentioned are dimethylacetamide, N-methylpyrrolidinone, dioxane, t-butanol, cumyl alcohol, isoamyl alcohol, tetramethylurea, diethylene glycol, halogenated hydrocarbons (e.g. chlorobenzene or dichlorobenzene) and ethers.
The quaternary salts used for the purposes of the present invention can be, for example, ammonium or phosphonium salts substituted by organic radicals. Suitable salts for use in the process of the invention are ammonium and phosphonium salts which bear C6-C10-aryl, C7-C12-aralkyl and/or C1-C20-alkyl radicals as organic radicals and a halide, tetrafluoroborate or hexafluorophosphate as anion. In the process of the invention, preference is given to ammonium salts which bear C6-C10-aryl, C7-C12-aralkyl and/or C1-C20-alkyl radicals as organic radicals and a halide as anion, particular preference being given to tetrabutylammonium bromide. The amount of such a quaternary salt is from 0.1 to 50% by weight, based on the weight of the reaction mixture. This amount is preferably from 0.5 to 15% by weight, particular preferably from 1 to 5% by weight.
The process of the invention is, preferably without solvent, carried out at from 30 to 200xc2x0 C., preferably at from 30 to 150xc2x0 C., particularly preferably at from 40 to 120xc2x0 C., and at a pressure of from 1 to 150 bar, preferably from 2 to 50 bar, particularly preferably at from 5 to 25 bar.
The supported catalysts can be used as powders or shaped bodies and can be separated again from the reaction mixture, for example by filtration, sedimentation or centrifugation.
The preparation of aromatic carbonates using the supported catalysts of the invention can be performed by means of different process variants. One possibility is a batchwise procedure. In the case of a continuous procedure in countercurrent or cocurrent or in the downflow mode over a fixed bed catalyst, space velocities of from 0.01 to 20 g of aromatic hydroxy compound per gram of supported catalyst and hour, preferably from 0.05 to 10 g of aromatic hydroxy compound per gram of supported catalyst and hour, particularly preferably from 0.1 to 5 g of aromatic hydroxy compound per gram of supported catalyst and hour, are set. The supported catalysts used in batchwise experiments can be repeatedly used for identical starting materials without purification. In the case of a continuous procedure, the supported catalysts used can remain in the reactor for a long time. Preferably, the supported catalysts of the invention are used in a continuous procedure in a single reactor or in a cascade of reactors.
If the supported catalyst is used as a powder, the stirred vessels to be used are fitted with stirrers suitable for mixing the reaction components. When working with supported catalyst powders in suspension in stirred vessels or bubble columns, amounts of from 0.001 to 50% by weight, preferably from 0.01 to 20% by weight, particularly preferably from 0.1 to 10% by weight, of supported catalyst powder are used, based on the amount of aromatic hydroxy compound used. In particularly preferred embodiments, the heterogeneous supported catalyst is used as a shaped body in a fixed position in stirred vessels, bubble columns, downflow reactors or cascades of these reactors; in this context, the various types of reactor may also occur simultaneously in one cascade.